Heretofore, various circuits have been designed for controlling load current in electrical load driving systems, wherein such circuits have typically been constructed of discrete electrical components, so-called hybrid circuits and integrated circuits. Oftentimes, particularly in the internal combustion engine industry, such circuitry is used in inductive load driving applications such as ignition control systems, fuel control systems and the like.
An example of one known ignition control system includes a low-valued sense resistor disposed in series with a coil current switching device which is itself series-connected to a low side of a primary coil forming part of an automotive ignition coil, wherein the opposite side of the primary coil is connected to a supply voltage. The coil current switching device may be, for example, an insulated gate bipolar transistor (IGBT) having a collector connected to the low side of the coil primary, a gate, and an emitter coupled to ground through the sense resistor. The IGBT is responsive to a gate drive signal to conduct coil current therethrough as is known in the art. As the coil current increases, a voltage is developed across the sense resistor, wherein this voltage is provided to an input of a closed-loop current control circuit operable to modulate the gate drive signal so as to limit and maintain the coil current at a desired coil current limit level. The coil current limit level guarantees sufficient energy in the ignition coil to create a spark for igniting the air/fuel mixture while preventing damage to, or destruction of, the ignition coil or IGBT due to excessive coil current levels.
One drawback associated with ignition control systems of the foregoing type is that the sense resistor must be constructed in such a manner that it is capable of withstanding the high coil current levels and corresponding power levels associated with the typical operation of an automotive ignition coil. This constraint requires a physically large resistor regardless of whether it is provided as a discrete, printed or integrated resistor. Moreover, since the voltage drop across the sense resistor adds to the voltage developed at the low side of the coil primary, the minimum supply voltage at which the ignition control system can achieve the desired coil current limit level is thereby increased. This condition is undesirable since automotive ignition control systems are typically required to be functional at very low battery voltages. Thus, to minimize voltage drop across the sense resistor, it must have a very low resistance value. Low-valued precision resistors, however, are expensive in both discrete and integrated form. Additionally, the power dissipation requirements of the sense resistor typically cause device heating that may lead to changes in the resistor value, ultimately resulting in undesirable corresponding changes in the coil current limit level.
To overcome at least some of the foregoing drawbacks, ignition control systems have heretofore been developed that implement a so-called "sense IGBT"; i.e., an IGBT having a second emitter configured to conduct an output current that is proportional to the "primary" emitter.
One particular example of a known ignition control system 10 implementing a sense IGBT is illustrated in FIG. 1. Referring to FIG. 1, ignition control system 10 includes ignition control circuitry 12 connected to a voltage source VBATT via signal path 14. In the application shown in FIG. 1, VBATT is a conventional automotive battery typically producing an output potential of approximately 14 volts. In any case, a voltage line VIGN is connected between ignition control circuitry 12 and one end of an ignition coil primary 18, wherein the ignition control circuitry 12 is typically operable to switchably provide the VBATT voltage on voltage line VIGN to thereby controllably provide a suitable voltage potential to the coil primary 18. The opposite end of the coil primary 18 is connected to one input of a suitable coil switching device such as, for example, the collector 28 of an IGBT 20. A gate 26 of IGBT 20 is connected to a gate drive output of ignition coil circuitry 12 via signal path 34, and a primary emitter 22 is connected to ground potential. A second "sense" emitter 24 of IGBT 20 is connected to a first end of a sense resistor R.sub.S 30, the opposite end of which is connected to ground potential. The first end of resistor R.sub.S is further connected to an input of known gate control circuitry 32, wherein an output of gate control circuitry 32 is connected to the base 26 of IGBT 20.
With VIGN=VBATT, ignition control circuitry 12 is operable to impress a gate drive voltage GD at the base 26 of IGBT 20. In response to the gate drive voltage GD, IGBT 20 is operable to turn on and conduct a coil current I.sub.L therethrough to ground potential via emitters 22 and 24. The sense emitter 24 is typically sized relative to the primary emitter 22 so that only 1-2% of the total coil current I.sub.L flows through the sense emitter with the remaining coil current IL flowing through the primary emitter 22. As the coil current I.sub.L increases through the inductive load of the coil primary 18, a voltage is developed across the sense resistor R.sub.S, wherein this voltage is supplied to the input of gate control circuitry 32. The gate control circuitry 32 forms a closed-loop current control circuit that is typically operable to compare the voltage drop across R.sub.S with a predefined reference voltage, and to control the gate drive voltage GD at a level sufficient to maintain the coil current I.sub.L at a desired current limit level when the voltage drop across R.sub.S reaches the predefined reference voltage.
Since only a small percentage of the total coil current I.sub.L flows through sense emitter 24, the "sense" current flowing through R.sub.S is much less than with the single emitter IGBT-based ignition control system described hereinabove. Accordingly, the sense resistor R.sub.S in system 10 may be larger in value, smaller in physical size and have less power dissipation capability than the sense resistor previously described herein. Such resistors can be easily created in integrated circuit form, thereby permitting R.sub.S to be fabricated on the same semiconductor device as the gate control circuitry 32.
An alternate use of an IGBT, such as IGBT 20, with a sense emitter, such as sense emitter 24, for limiting current through a load is described in U.S. Pat. No. 5,396,117 to Housen et al. The Housen et al. circuit is described as having two modes of operation. In a first mode, "on/off" circuitry is provided that turns the IGBT completely off if a sense current flowing through the sense emitter and sense resistor connected thereto exceeds a predetermined value, thereby providing over-current protection capability. In a second mode, short circuit detection circuitry is provided that steps the IGBT gate drive voltage down to a fixed voltage level, defined by a zener diode breakdown voltage, upon detection of a short circuited load condition. It is important to note, however, that the Housen et al. circuitry does not attempt to otherwise modulate the IGBT gate voltage in a manner that would allow for stable, dynamic current limiting/maintaining of an inductive load.
In any case, while the ignition control system 10 illustrated in FIG. 1 overcomes some of the problems associated with the single-emitter IGBT ignition control system previously described hereinabove, system 10 has certain drawbacks associated therewith. For example, as with dynamic current limit control of any electrical load, and with inductive loads in particular, the control of sense current flowing through sense emitter 24 and resistor R.sub.S is subject to the possibility of loop instability and subsequent oscillation of the load current I.sub.L. Moreover, an inherent characteristic of the sense IGBT device 20 further complicates this issue. When the voltage across the collector and emitter terminals (Vce) of a sense IGBT 20 increases, the ratio of current through the sense emitter 24 to the current through the primary emitter 22 also increases, thereby causing the current through the sense emitter 24 to become a larger percentage of the total current I.sub.L. In a load current limit control system such as system 10 illustrated in FIG. 1, the IGBT 20 is initially driven with a gate drive voltage GD that is sufficient to drive the IGBT into saturation, thereby resulting in Vce voltages that are low (typically less than 2 volts) relative to the supply voltage VIGN. When the coil current I.sub.L approaches the desired limit level, the gate control circuitry 32 reduces the gate drive voltage GD which causes Vce to increase, thereby causing the coil current I.sub.L to remain constant. Since the control of the coil current I.sub.L is a function of the ratio of the sense emitter current to the primary emitter current, the resulting change in this ratio due to changes in Vce causes perturbations in the gate drive control circuitry 32. These perturbations can lead to oscillation of the coil current I.sub.L, wherein such oscillations can be sufficiently severe so as to generate voltages on the secondary coil windings that are high enough to generate a spark event at an associated spark plug. Automotive ignition systems generally require precisely controlled timing of spark events in the engine cylinders, and any oscillation of the ignition coil current I.sub.L during certain critical time periods can cause premature spark events, resulting in rough engine operation, poor emission control and/or engine damage.
What is therefore needed is an improved ignition coil control system for use with a multiple output load driving device that does not suffer from the foregoing drawbacks of known ignition coil control systems.